1. Field of the Invention
This invention relates generally to temperature sensing apparatus, and more particularly the invention relates to temperature sensing apparatus which utilizes temperature sensitive acoustical resonators.
2. Description of the Prior Art
Various electronic apparatus is available for remotely sensing temperature. However, in some industrial applications electrical and electronic sensors are not feasible because of the particular operating environment. An example is in high power electrical transformers where operating temperature can be critical but where electrical sensors, such as thermisters or radiowave sensors, for example, could have deleterious effects.
The use of acoustical resonators in temperature sensors for such applications is known and has been successfully employed. A temperature sensitive acoustical resonator is placed in the environment whose temperature is to be sensed, and acoustical waves are directed to the resonator. By sensing the frequency of the resonant acoustical wave from the resonator, the temperature of the environment can be known since the resonant frequency is temperature dependent. One such system employs a temperature sensor similar to the sensor described in U.S. Pat. No. 3,318,152 issued to Thompson et al.
Such systems employ resonators having a single mode flexural vibration with two nodal diameters. By detecting the mechanical resonant frequency of the resonator, the temperature can be measured. For a titanium sensor the resonant frequency decreases with increasing temperature approximately 300 parts per million per degree celcius. However, other fixed vibration modes may exist because of extraneous standing waves on waveguides and resonances within the receiving and sending transducers. Such spurious resonances can result in a faulty temperature reading which can lead to improper operation of industrial apparatus such as large electrical transformers.